2011 (SBTE) 25th Annual Meeting Proceedings - International ...

R.C. Cheb
hebel.
el. 2011. Use of Applied Reproductive Technologies (FTAI, FTET) to Improve the Reproductive Efficiency in
Dairy Cattle. Acta Scientiae Veterinariae. 39(Suppl 1): s203 - s221.
[20]. Whether this is the case in the porcine embryo
is not known. The mesodermal cells arrange
themselves as an intermediate cell layer between the
two developing epithelial layers, i.e. the epiblast and
endoderm.
The cells entering the primitive streak are
exposed to distinct signaling factors at different concentrations
dependent on where in the primitive streak
the cells ingress through. Cell tracing studies has
shown that the fate of a given cell is related to the site
of ingression through the primitive streak: Cells ingressing
through anterior streak and node become
prechordal plate mesoderm, notochord, and endoderm,
cells ingressing through “mid” streak become
paraxial mesoderm, and cells migrating through the
posterior streak become extra-embryonic and lateral
plate mesoderm. These cell movements are, beside
geometrical differences, being conserved from
reptiles to mouse [24].
When the primitive streak has reached its
maximum extension of about two thirds of the length
of the embryonic disc, a subsequent posterior
retraction the streak occurs. Until recently, it was
thought that the primitive streak actually shortened
during this retraction. However, new investigations
in the mouse have shown that the node does not
regress posteriorly, but that the streak becomes
relatively shorter due to the longitudinal growth of
the embryonic disc (Yamanaka et al., 2007). Along
with this process, cells ingressing through the node
forms the notochord; a rod-shaped structure
interposed between the epiblast and the endoderm
extending from the rostral end of the embryonic disc
posteriorly to the node, from which it grows in a posterior
direction (Figure 10). The notochord posterior
to the node is apparently formed by particular cells
migrating posteriorly from the node [48]. The
notochord expresses BRACHYURY [45].
With the formation of the three somatic germ
layers; ectoderm, mesoderm, and endoderm and the
PGCs (see later), the progenitors of all fetal tissue
lineages are formed.
IV. NEURULATION: DEVELOPMENT OF THE NEURAL
ECTODERM AND NEURAL CREST
Neurulation is the process leading to the
formation of the neural tube, the precursor of the central
nervous system including the brain and spinal
Figure 10. Confocal laser scanning micrographs of Day 14 porcine embryo. (A) Dorsal view of the embryonic disc showing the epiblast and
developing ectoderm, identified from persisting OCT4 expression, and T (BRACHYURY) expression in the primitive streak (PS) posteriorly,
in the node (N), and in the notochord (No) anteriorly. Note the cluster of OCT4 expressing primordial germ cells posteriorly (arrow). (B) Optical
side view of the embryonic disc displaying the same features. A: Anterior; P: Posterior; D:Dorsal; V: Ventral. Modified from Wolf et al.
(2011b)[45].
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